1. Field of the Invention
The present invention relates to a light-emitting device using a thin film that is made of a luminous material. Further, the present invention relates to electric equipment using the light-emitting device as a display portion or a light source.
2. Description of the Related Art
In recent years, development is proceeding in a light-emitting device (hereinafter referred to as an EL light-emitting device) employing a luminous element (hereinafter referred to as an EL element) that uses a thin film (hereinafter referred to as an EL film) made of a luminous material that provides EL (Electro Luminescence). The EL device is called a light emitting device or a light emitting diode or OLED (Organic Light Emission Diode). The EL (electroluminescent) devices referred to in this specification include triplet-based light emission devices and/or singlet-based light emission devices, for example. The EL light-emitting device has an EL element that is composed of an anode, a cathode, and an EL film sandwiched therebetween. The emission of light can be attained from the EL light-emitting device by applying a voltage between the anode and the cathode. In particular, an organic film that is used as the EL film is referred to as an organic EL film. Note that a luminous material in which EL can be obtained includes a luminous material that luminesces via a singlet excitation and a luminous material that luminesces via a triplet excitation.
A metal that has a small work function (typically a metal belonging to Group 1 or Group 2 of the periodic table) is mostly used as the cathode, and a transparent oxide conductive film such as a compound film of indium oxide and tin oxide (ITO) is mostly used as the anode. Therefore, the emission of light attained is visible after the light is transmitted through the anode.
Recently, development is proceeding in an active matrix type EL light-emitting device in which the control of the emission of light by the EL elements provided in respective pixels is through the use of a TFT (thin film transistor), and the development thereof has reached a stage where trial products have been released. All these trial products use a pixel electrode as the anode, and hence the structures thereof are such that the light generated by the EL elements is irradiated to the side of the TFT.
However, because light is no transmitted to the regions where the TFTs and wirings are formed in such structures, a light emitting area (hereinafter referred to as an effect light emitting area) that can actually be seen is reduced drastically. Therefore, the necessity of raising the luminance of the light emitted in order to obtain a bright image leads to the result of hastening the deterioration of the organic EL film.
The present invention has been made in order to solve the above problem, and therefore has an object to provide a bright and highly reliable light-emitting device. Further, another object of the present invention is to provide highly reliable electric equipment that uses the light-emitting device as its display portion or light source.
The present invention is characterized in the employment of an EL element 100 having a structure shown in FIG. 1. In FIG. 1, reference numeral 101 denotes a reflecting electrode that is made of a metallic film. It is preferable that a metallic film that has a high reflectance is used as the reflecting electrode 101. An aluminum film (aluminum alloy film or an aluminum film containing a dopant) or a silver thin film may be used. In addition, the conductive film performed by aluminum plating or silver plating may also be used as the reflecting electrode 101.
Next, reference numeral 102 denotes an anode of the EL element 100 which is made of a transparent conductive film (hereinafter referred to as a transparent conductive film) with respect to visible radiation. It is to be noted that transparency with respect to the visible radiation (light of visible radiation region) indicates that the visible radiation transmits at a transmittance of between 80% and 100%. In case of using an oxide conductive film (typically a compound film of indium oxide and tin oxide or a compound film of indium oxide and zinc oxide) as the transparent conductive film, it is preferable that the film thickness thereof is formed between 10 and 200 nm (preferably between 50 and 100 nm).
At this point, a work function of the anode 102 determines a hole injection barrier, and the reflecting electrode 101 reflects the light emitted from the EL element and applies a uniform voltage to the anode 102 at the same time.
Next, reference numeral 103 denotes an EL layer. The EL layer 103 includes an EL film that has a single layer or multiple layers. It is to be noted that the EL film may be an organic EL film or an inorganic EL film, or it may be formed by laminating those films. Further, the structure of the EL layer 103 may be any known structure. In other words, throughout this specification, the EL layer is a layer formed by freely combining an electron injection layer, an electron transport layer, and an EL film (also referred as a light-emitting layer). Of course, the EL film may be a low molecular weight or a high molecular weight film.
Reference numeral 104 denotes a cathode of the EL element 100. A metallic film having a small work function (about xe2x88x923.5 to xe2x88x923.8 eV) is used as the cathode 104. Ametallic film containing an element that belongs to Group 1 or Group 2 of the periodic table may be used as the metallic film having such a work function. Therefore, in the present invention, it is desirable that a 10 to 70 nm thick (preferably between 20 and 50 nm) metallic film containing an element that belongs to Group 1 or Group 2 of the periodic table is used as the cathode 104.
Visible radiation can be transmitted through such a metallic film as the above, which has a thin film thickness. Thus, the cathode 104 can be used as a transparent electrode to visible radiation.
Next, reference numeral 105 denotes an electrode, which is made of the transparent conductive film, in contact with the cathode (hereinafter referred to as an auxiliary electrode). An oxide conductive film typified by a compound film of indium oxide and tin oxide or a compound film of indium oxide and zinc oxide may be used as the auxiliary electrode 105. The film thickness thereof may be formed to between 10 and 200 nm (preferably between 50 and 100 nm). At this point, a work function of the cathode 104 determines the hole injection barrier, and the auxiliary electrode 105 applies a uniform voltage to the cathode 104.
When the EL element has the above-described structure, the light generated at the EL layer (strictly the EL film contained in the EL layer) can be observed from the side of the auxiliary electrode 105 (the upper direction in FIG. 1). This fact can be easily comprehended by considering that the light advancing to the side of the anode 102 is mostly reflected by the reflecting electrode 101.
An effect of the present invention is in that the extraction of the emission of light of the EL light-emitting device from the side of the cathode, which in the prior art had been difficult, can now be carried out with ease. This effect is particularly remarkable during the formation of the active matrix type EL light-emitting device.